Gas sensors based on the infrared optical measurement principle are known from the state of the art and are used to monitor gas concentrations in industry for many different applications. The gas sensors detect increased concentrations of explosive gases or gases harmful to the human body, for example, methane (CH4) and carbon dioxide (CO2) and are integrated in warning and alarm systems.
An infrared optical gas sensor, in which the gas enters the sensor housing via a perforated plate, is known from WO 2005/054827. The perforated plate represents an upper limitation of a gas-filled part of the sensor housing, into which the gas to be measured enters via the perforations formed in the perforated plate. A radiation source and an optical detector are arranged in the interior of the gas sensor in a bottom part, which represents the lower limitation of the gas-filled part of the sensor housing.
The IR radiation is sent by the radiation source and is reflected at the perforated plate towards the optical detector. The intensity reaching the detector is an indicator of the measured gas concentration based on the optical absorption caused by the gas. The gas species is selected and the attainable measuring sensitivity and measuring accuracy are determined by the selected wavelength range of the infrared radiation combined with the dimensions of the gas-filled part of the sensor housing, the so-called measuring gas cell.
The gas entry shown in WO 2005/054827 via a perforated plate allows the measuring gas to enter the measuring gas cell via the total area of the perforations in the perforated plate, on the one hand, and, on the other hand, part of the emitted light is lost through the perforations in the perforated plate in the measuring environment, so that this part of the light cannot be reflected onto the detector. The selection of the dimension, position and quantity of perforations in the perforated plate defines, on the one hand, the loss of light to the measuring environment but, on the other hand, also the velocity of entry of the measuring gas into the measuring gas cell.
WO 2007/091043 A1 discloses a gas sensor, whose measuring gas cell is spherical for guiding the propagation of the optical rays, wherein the guiding of the optical radiation brings about focusing of the rays to the detector and thus increases the intensity of the light falling on the detector.
The extent of loss of light, both through the gas inlet opening and due to the absorption in the gas, determines the intensity of the radiation at the detector. The velocity at which the measuring gas enters the measuring gas cell determines the rise time of the gas sensor, which is often called the t10-90 time and provides information on the time at which a measured value rises from a level of 10% of the final value to a level corresponding to 90% of the final value.